Background

A. LEO Satellite Nework

In recent years, LEO satellite networks have drawn widespread attentions due to the rapid advance in manufacturing and launch technologies [16]. Based on the large-scale deployment of LEO satellites, UEs on the ground can enjoy low-latency and high-bandwidth network services with global coverage. Actually, LEO satellite networks can complement and integrate with the traditional terrestrial networks to provide more robust and disaster-resistant communication services. Up to now, it is reported that there have been millions of users using services provided by satellite networks [17].

To achieve real global coverage, ISL have been introduced and testing. In convention, each LEO satellite builds four ISLs with nearby satellites, respectively as two intra-orbit links that connect to the preceding and succeeding satellites in the same orbit and two inter-orbit links that connect to the satellites in adjacent orbits. Based on the ISLs using laser communications, high-bandwidth and stable communication between satellites can be easily achieved.

近年来，得益于制造和发射技术的飞速发展，低地球轨道（LEO）卫星网络引起了广泛关注[16]。基于LEO卫星的大规模部署，地面用户终端（UEs）可以享受到具有全球覆盖、低延迟和高带宽特性的网络服务。实际上，LEO卫星网络能够作为传统地面网络的补充并与之融合，以提供更具鲁棒性和抗灾能力的通信服务。据报道，迄今为止已有数百万用户正在使用卫星网络所提供的服务[17]。

为了实现真正的全球覆盖，星间链路（ISL）技术已被引入并展开测试。通常情况下，每颗LEO卫星与其邻近卫星建立四条星间链路，分别为：两条轨道内链路，用于连接同一轨道内的前、后两颗卫星；以及两条轨道间链路，用于连接相邻轨道上的卫星。基于使用激光通信的星间链路，可以轻松实现卫星之间的高带宽和稳定通信。

B. Handover in Mobile Satellite Network

Mobile satellite network follows the handover procedure specified in 5G standard [7]. There exist two kinds of handover schemes, respectively as Xn-based and N2-based schemes [2]. By leveraging ISLs, Xn-based handover strategy can provide lower handover latency compared to the N2-based one. High-level Xn-based handover procedure can be summarized as follows. First, UE disconnects the built Radio Resource Control (RRC) link with the source S-gNB and establishes a new RRC satellite-ground connection with the target S-gNB. Secondly, the target S-gNB delivers the handover results to the User Plane Function (UPF) through the ISLs and satelliteground link, which is a critical part of the core network.

卫星移动网络遵循5G标准[7]中规定的切换流程。切换方案主要存在两种，分别为基于Xn接口的切换和基于N2接口的切换[2]。通过利用星间链路（ISLs），基于Xn的切换策略相比基于N2的策略，能够提供更低的切换延迟。高层级的Xn切换流程可概括如下：首先，用户终端（UE）断开与源卫星基站（source S-gNB）已建立的无线资源控制（RRC）链路，并与目标卫星基站（target S-gNB）建立新的RRC星地连接。其次，目标S-gNB通过星间链路和星地链路，将切换结果传送至作为核心网关键部分的用户平面功能（UPF）实体。

According to the above framework, there exists a key challenge (i.e., the handover problem) in mobile satellite network. Typically, handover in mobile satellite network differs from the terrestrial network in two main aspects—which will be illustrated as below.

在上述框架下，卫星移动网络存在一个关键挑战，即切换问题。通常，卫星移动网络中的切换与地面网络在以下两个主要方面存在差异:

First, ground UEs experience frequent handover due to the fast travelling speed and small coverage of LEO satellites (compared with GEO satellites). In average, there exists a handover happening every 3 minutes. Actually, the handover interval may be even shorter because of the weather and UE’s access strategy. Table I depicts the orbit information for two well-known commercial constellations. From this table, we can see that LEO satellites operating at different altitudes have different speeds and thus different time duration for connection. For both of these two constellations, the handover interval is less than 3 minutes because of the limited coverage time for each satellite.

首先，由于低地球轨道（LEO）卫星的移动速度快且单个卫星的覆盖范围小（与地球静止轨道（GEO）卫星相比），地面用户终端会经历频繁的网络切换。平均而言，每3分钟就会发生一次切换。实际上，考虑到天气状况和用户终端的接入策略，切换间隔可能更短。表I描述了两个知名商业卫星星座的轨道信息。从该表中可以看出，运行在不同高度的LEO卫星具有不同的速度，因此其单次连接的持续时间也不同。对于这两个星座而言，由于单颗卫星的覆盖时间有限，其切换间隔均短于3分钟。

Secondly, another distinguishing feature from the terrestrial network is that the mobile satellite network can provide seamless coverage all over the world even in deserts or oceans, which implies that the distance between UEs and the core network can be much longer than the terrestrial network [4], [18]. Consequently, ISLs should be utilized to achieve handover signalling delivery between the access and core networks, which brings much higher latency. For example, Fig. 2 describes the signaling delivery between Shanghai and London, the distance of which reaches up to 9,000 km. Obviously, this incurs a rather long transmission latency, as well as a large handover latency. Moreover, due to the limited number of ISLs that can be established for each satellite, circuitous transmission path between the access and core networks are common with more ISLs involved, thereby prolonging the propagation and handover latency.

其次，卫星移动网络的另一个显著特征是它能够为全球提供无缝覆盖，即使在沙漠或海洋等地区。这意味着用户终端与核心网之间的距离可能远超地面网络[4], [18]。因此，必须利用星间链路来实现接入网络与核心网之间的切换信令传输，但这会带来更高的延迟。例如，图2描述了上海和伦敦之间的信令传输，两地距离长达9,000公里。显然，这会导致相当长的传输延迟和切换延迟。此外，由于每颗卫星能够建立的星间链路数量有限，接入网络与核心网之间的传输路径通常较为迂回，需要经过更多的星间链路，从而进一步增加了传播延迟和切换延迟。